The present invention relates generally to implantable medical devices, and specifically to implantable pressure sensors.
A significant effort has been underway for many years to develop implantable medical devices for direct measurement of physiological parameters of a patient for both temporary and chronic use. Some prior art devices cannot be implanted in the body due to their undesirably large size, limited life span, or high power consumption Challenges exist, for example, in developing efficient commercial pressure transducers, capable of being used in the body of a patient for direct measurement of physiologic pressures such as urinary bladder, abdominal, respiratory, cardiac, venous, arterial, amniotic, and cerebrospinal fluid pressures. For example, suitable implantable cardiac pressure sensors which have very low power consumption for tracking the condition of a heart failure patient are not available.
U.S. Pat. No. 6,248,083 to Smith et al., U.S. Pat. No. 5,969,591 to Fung, U.S. Pat. No. 5,566,680 to Urion et al., U.S. Pat. No. 5,522,266 to Nicholson et al., U.S. Pat. No. 5,184,619 to Austin, U.S. Pat. No. 4,873,986 to Wallace, and U.S. Pat. No. 4,825,876 to Beard, which are incorporated herein by reference, describe the use of piezoresistive elements to facilitate pressure measurements for medical applications.
U.S. Pat. No. 4,407,296 to Anderson, which is incorporated herein by reference, describes a hermetically-sealed piezoresistive pressure transducer for implantation in the body.
U.S. Pat. No. 4,846,191 to Brockeway et al., which is incorporated herein by reference, describes an implantable device for chronic measurement of internal body pressures. The device may include a piezoresistive element.
U.S. Pat. No. 4,023,562 to Hynecek et al., which is incorporated herein by reference, describes an implantable piezoresistive pressure transducer for monitoring internal fluid or pneumatic pressures within the body.
U.S. Pat. No. 4,432,372 to Monroe, which is assigned to Medtronic, Inc. and is incorporated herein by reference, describes apparatus for multiplexing the power and signal leads of an implantable piezoelectric pressure transducer. A device built according to the description in the Monroe patent charges a capacitor located at the pressure transducer site, and, subsequently, allows the capacitor to discharge through a Wheatstone bridge. The capacitor is a fundamental component of this device, as it permits the time-sharing of power functions and sensing functions into only two wires. Although not stated in the Monroe patent, it is known that repeated charging and discharging of capacitors is often associated with significant heat dissipation, and, therefore, increased energy consumption.
U.S. Pat. No. 6,221,024 to Miesel, which is also assigned to Medtronic, Inc. and is incorporated herein by reference, describes a method for sealing oil-filled pressure transducer modules for a chronically-implantable pressure sensor lead. The ""024 patent states: xe2x80x9cU.S. Pat. No. 4,023,562 describes a pressure transducer comprising a piezoresistive bridge of four, orthogonally disposed, semiconductor strain gauges formed interiorly on a single crystal silicon diaphragm area of a silicon base. A protective silicon cover is bonded to the base around the periphery of the diaphragm area to form a sealed, evacuated chamber. Deflection of the diaphragm due to ambient pressure changes is detected by the changes in resistance of the strain gauges. Because the change in resistance is so small, a high current is required to detect the voltage change due to the resistance change. The high current requirements render the piezoresistive bridge unsuitable for long term use with an implanted power source. High gain amplifiers that are subject to drift over time are also required to amplify the resistance-related voltage change.xe2x80x9d
U.S. Pat. No. 5,564,434 to Halperin et al., which is incorporated herein by reference, describes an endocardial lead for implantation in a heart chamber. The lead is able to sense pressure changes via capacitors, and transmit information responsive to the pressure changes to an internal or external medical device.
U.S. Pat. No. 5,330,505 to Cohen, which is incorporated herein by reference, describes implantable sensors for sensing a variety of cardiac physiologic signals.
U.S. Pat. No. 6,238,423 to Bardy, which is incorporated herein by reference; describes apparatus for treating chronic constipation, which includes an implantable pressure sensor for sensing tension in a wall of the digestive system.
U.S. Pat. No. 6,240,316 to Richmond et al., which is incorporated herein by reference, describes the use of implanted pressure sensors in apparatus for treating sleep apnea.
Measurement of pressure in the vicinity of the bladder and lower abdominal region is an important element in devices and methods for treating and controlling urinary incontinence. U.S. Pat. No. 6,135,945 to Sultan, which is incorporated herein by reference, describes apparatus for preventing urinary incontinence. The described apparatus includes an implanted pressure sensor for sensing intra-abdominal pressure. U.S. Pat. No. 4,571,749 to Fischell, which is incorporated herein by reference, describes an artificial sphincter device whose pressure can vary in response to changes in abdominal or intravesical (bladder) pressure. U.S. Pat. No. 5,562,717 to Tippey et al., which is incorporated herein by reference, describes electrical stimulation treatment for incontinence and other neuromuscular disorders, and includes a pressure sensor for determining changes in pressure in the vaginal or anal muscles.
PCT Patent Publication WO 00/19939 to Gross et al., entitled xe2x80x9cControl of urge incontinence,xe2x80x9d which is assigned to the assignee of the present patent application and incorporated herein by reference, describes a device for treatment of urinary urge incontinence comprising a system in which imminent involuntary urine flow is sensed, and appropriate nerves or muscles are stimulated to inhibit the flow.
U.S. Patent Publication WO 00/19940 to Gross et al., entitled, xe2x80x9cIncontinence treatment device,xe2x80x9d which is assigned to the assignee of the present patent application and incorporated herein by reference, describes a device for treating urinary stress incontinence comprising a system in which imminent involuntary urine flow is sensed, and nerves or muscles are stimulated to inhibit the flow.
In general, for implanted pressure sensors, issues of size, durability, accuracy and, particularly, power consumption are major considerations that must be addressed in order to ensure that the goals of the application are achieved.
It is an object of some aspects of the present invention to provide improved apparatus and methods for reducing power consumption in a pressure sensor implanted in the body of a patient.
It is also an object of some aspects of the present invention to provide improved methods and apparatus for reducing heat dissipation in a pressure sensor implanted in the body of a patient.
It is a further object of some aspects of the present invention to provide improved apparatus and methods for increasing the useful lifetime of an implantable device.
It is yet a further object of some aspects of the present invention to provide improved methods for enabling the coupling of MP35N wires and other materials in lead wires to circuitry in an implantable device.
It is still a further object of some aspects of the present invention to provide less-complicated methods and apparatus for producing an implantable device.
It is also an object of some aspects of the present invention to provide improved methods and apparatus for producing a low cost implantable device.
In preferred embodiments of the present invention, apparatus to achieve the above objects comprises at least one implantable pressure-sensing device coupled to a control unit. The sensing device is preferably implanted in a patient""s body at a location chosen in accordance with the particular condition being treated or diagnostic procedure being performed. Preferably, the sensing device comprises an element characterized by electrical resistance that varies as a function of the pressure imposed upon it, typically a piezoresistive element. The pressure measuring apparatus is preferably designed such that the piezoresistive element of the sensing device is integrated into a Wheatstone bridge electrical circuit as one of the four resistors (typically adapting techniques described in one or more of the references cited hereinabove and incorporated herein by reference). Alternatively, two or more of the resistors in the Wheatstone bridge include piezoresistive elements. The sensing device operates by receiving a designated driving signal from the control unit and, as a function of the pressure upon it, undergoing a change in resistance that causes a measurable variation in the voltage output of the Wheatstone bridge. The driving signal includes a series of relatively short duration, low duty cycle pulses.
Consequently, in preferred embodiments of the present invention, the actual time during which the apparatus consumes power by driving current through the Wheatstone bridge and taking measurements is significantly less than the total time of operation of the apparatus. In prior art implantable piezoresistive pressure sensors, by contrast, a major shortcoming is high power consumption, which limits usable lifetime and imposes demanding requirements upon power supplies, such as batteries.
Electrical leads for implantable medical devices are often composed of MP35N or platinum/iridium (Pt/Ir), or, less commonly, alloys having iron in low quantities (e.g., moderately or significantly less than 60% iron by weight), as these materials have proven to be both safe and effective for many applications in the human body. A problem with using MP35N (or these other materials) for electrical leads is that it does not solder well to copper, which is a common conductor in electrical circuits. In particular, increasing iron content is associated with increased facility in soldering, but decreased biocompatibility. MP35N and Pt/Ir, having essentially no iron, are particularly difficult to solder using standard techniques. The following solution, and that elaborated more completely in the Detailed Description of Preferred Embodiments, while described with respect to MP35N by way of illustration, applies as well to platinum/iridium and other alloys having low iron content (e.g., 1-60% iron, 1-40% iron, or 1-20% iron). Additionally, it applies to DFT wire (Fort Wayne Metals, Fort Wayne, Ind.) and other similar types of lead wires, in which a highly-conductive core is surrounded by a less conductive, more biocompatible outer surface. For example, the techniques described herein may be applied to a lead wire having MP35N over a silver core.
In preferred embodiments of the present invention, MP35N wires are used to connect the control unit to a circuit in an implanted pressure sensor or other medical device. The problem of securely connecting the MP35N wire to the circuit board is overcome by using an intermediate conductor to couple MP35N wires to the circuit. In some preferred embodiments, a stainless steel cylinder is mechanically coupled to an MP35N wire, for example by crimping. The stainless steel cylinder is then soldered to the circuit board. In other preferred embodiments, the cylinder comprises other biocompatible conductors, suitable for being mechanically coupled to the MP35N wire and for soldering to the circuit board. In further preferred embodiments of the present invention, the solder used to couple the MP35N wire and the circuit board comprises indium, preferably a high percentage of indium, as the inventors have found that this facilitates good electrical coupling of the MP35N wire and the circuit board, even without the use of stainless steel cylinders.
In order for the electrical circuit comprising the pressure measuring apparatus to function properly inside the human body, it must be protected from the generally electronics-incompatible environment at the implant location. Additionally, the device must be robust enough to survive the implant procedure. Thus, in preferred embodiments of the present invention, the electrical circuit in the pressure measuring apparatus and the connections to the MP35N wires are secured inside a hollow stainless steel tube or other casing comprising a sensing hole through which pressure changes can be measured. Preferably, the tube is filed with a generally-incompressible biocompatible substance that efficiently conveys pressure changes, such as a silicon gel. The tube has one or more gel-transport holes through which the gel can be added, while excess gel is forced through the sensing hole and/or the other gel-transport holes, such that all air bubbles are forced out of the tube. Optionally, the gel-transport holes may include the sensing hole. It is important to remove any air bubbles in order to obtain accurate pressure measurements, as air bubbles in the gel reduce the sensitivity of the pressure sensor.
Preferably, the sensor is coated with a protective substance, such as parylene, to protect it from external moisture. In a preferred embodiment, the stainless steel tube is further encased in a flexible tube, such as one made of silicone, which is able to convey pressure fluctuations through the sensing hole to the pressure sensor, while maintaining the integrity of the sensor and the gel.
There is therefore provided, in accordance with a preferred embodiment of the present invention, pressure-measuring apparatus, including:
a battery;
a pressure transducer, which is adapted to be placed in a patient, the pressure transducer having a characteristic mechanical response bandwidth f, and a corresponding mechanical response period p equal to 1/f; and
a control unit, which is adapted to actuate the battery to drive current through the pressure transducer for a current-driving time period less than 0.5 p, and to sense an electrical characteristic of the pressure transducer during the current-driving time period.
Typically, the pressure transducer is adapted to be implanted in the patient. Alternatively, the pressure transducer is adapted to be incorporated in a catheter.
For some applications, the pressure transducer is adapted to measure an abdominal pressure of the patient, a pressure of a urinary bladder of the patient, a cardiac pressure of the patient, or a blood pressure of the patient.
The pressure transducer preferably includes a piezoresistive pressure transducer, incorporated in a Wheatstone bridge circuit.
In a preferred embodiment, the control unit is adapted to set the current-driving time period to be less than 1000 microseconds. The control unit is typically adapted to designate an initial portion of the current-driving time period as a pressure-transducer stabilization period, during which the control unit withholds from sensing the characteristic.
As appropriate, the control unit may be adapted such that, in sensing the electrical characteristic, the control unit senses a current passing through the pressure transducer and/or a voltage drop across two points of the pressure transducer. The control unit is preferably adapted to sense the electrical characteristic substantially only during the current-driving time period.
Preferably, the control unit is adapted to actuate the battery to expend less than 5 microjoules in driving the current through the pressure transducer. Moreover, the control unit is preferably adapted to actuate the battery to drive the current directly into the pressure transducer, substantially without charging a capacitor located at a placement site of the pressure transducer. In particular, the control unit is preferably adapted to actuate the battery to drive the current directly into the pressure transducer, substantially without charging a capacitor located at a placement site of the pressure transducer having capacitance greater than 0.1 nF.
Additionally, in a preferred embodiment, the control unit is adapted to actuate the battery to drive current into the pressure transducer during a plurality of current-driving time periods, each less than 0.5 p, and to sense respective electrical characteristics of the pressure transducer during each of the current-driving time periods. In this case, the battery is preferably adapted to drive the current directly into the pressure transducer, substantially without charging a capacitor located at a placement site of the pressure transducer during each of the current-driving time periods.
For some applications, the control unit is adapted to actuate the battery to drive current through the pressure transducer during a plurality of current-driving time periods, each less than 0.5 p. Additionally, a duty cycle of the control unit (defined by a length of one of the current-driving time periods divided by a time between the initiation of two successive current-driving time periods) is preferably less than 0.3%, or even less than 0.03%.
In a preferred embodiment, the apparatus includes a signal processor, adapted to be placed in the patient at a common placement site with the pressure transducer and to process the sensed electrical characteristic. For example, the signal processor may include an amplifier, adapted to amplify the sensed electrical characteristic. Alternatively or additionally, the signal processor includes a microprocessor. In this case, the apparatus preferably includes:
a first set of wires, adapted to couple the control unit to the microprocessor; and
a second set of wires, adapted to couple the microprocessor to the pressure transducer,
wherein the number of wires in the second set of wires is greater than the number of wires in the first set of wires.
As appropriate, the control unit may be adapted to set the current-driving time period to be less than 0.1 p, less than 0.02 p, or even less than 0.004 p.
In some preferred embodiments of the present invention, the control unit is adapted: (a) to actuate the battery to drive current through the pressure transducer during a plurality of current-driving time periods, each less than 0.5 p, (b) to sense respective electrical characteristics of the pressure transducer during each of the current-driving time periods, and (c) to space the current-driving time periods by at least ten milliseconds. For some applications, the control unit is adapted to space the current-driving time periods by at least one second, by at least one minute, or even by at least one hour.
There is further provided, in accordance with a preferred embodiment of the present invention, pressure-measuring apparatus, including:
a pressure transducer, which is adapted to be placed at a pressure-sensing site in a patient, the pressure transducer having a characteristic mechanical response bandwidth f, and a corresponding mechanical response period p equal to 1/f; and
a control unit, adapted to be placed at a control-unit site at least 3 cm from the pressure-sensing site, to drive current through the pressure transducer for a current-driving time period less than 0.5 p, and to sense an electrical characteristic of the pressure transducer during the current-driving time period.
In a preferred embodiment, the control unit is adapted to be placed at a control-unit site which is at least 5 cm from the pressure-sensing site.
There is still further provided, in accordance with a preferred embodiment of the present invention, pressure-measuring apparatus, including:
a battery;
a pressure transducer, which is adapted to be placed in a patient; and
a control unit, which is adapted to actuate the battery to drive current through the pressure transducer for a current-driving time period less than 1000 microseconds, and to sense an electrical characteristic of the pressure transducer during the current-driving time period.
For some applications, the control unit is adapted to set the current-driving time period to be less than 250 microseconds, less than 50 microseconds, less than 10 microseconds, or even less than 2 microseconds.
There is yet further provided, in accordance with a preferred embodiment of the present invention, pressure-measuring apparatus, including:
a pressure transducer, which is adapted to be placed at a pressure-sensing site in a patient; and
a control unit, adapted to be placed at a control-unit site at least 3 cm from the pressure-sensing site, to drive current through the pressure transducer for a current-driving time period less than 1000 microseconds, and to sense an electrical characteristic of the pressure transducer during the current-driving time period.
There is also provided, in accordance with a preferred embodiment of the present invention, a method for measuring pressure via a pressure transducer, placed in a patient, the pressure transducer having a characteristic mechanical response bandwidth f, and a corresponding mechanical response period p equal to 1/f, the pressure transducer being coupled to a battery, the method including:
actuating the battery to drive current through the pressure transducer for a current-driving time period less than 0.5 p; and
sensing an electrical characteristic of the pressure transducer during the current-driving time period.
There is additionally provided, in accordance with a preferred embodiment of the present invention, a method for measuring pressure via a pressure transducer, placed in a patient at a pressure-sensing site, the pressure transducer having a characteristic mechanical response bandwidth f, and a corresponding mechanical response period p equal to 1/f, the method including:
from a control-unit site at least 3 cm from the pressure-sensing site, driving current through the pressure transducer for a current-driving time period less than 0.5 p; and
sensing an electrical characteristic of the pressure transducer during the current-driving time period.
There is still additionally provided, in accordance with a preferred embodiment of the present invention, a method for measuring pressure via a pressure transducer, placed in a patient, the pressure transducer being coupled to a battery, the method including:
actuating the battery to drive current through the pressure transducer for a current-driving time period less than 1000 microseconds; and
sensing an electrical characteristic of the pressure transducer during the current-driving time period.
There is yet additionally provided, in accordance with a preferred embodiment of the present invention, apparatus which is adapted to be placed in a patient, including:
circuitry, which is adapted to be placed in a patient;
a lead wire; and
an electrically-conductive connector, which is crimped to the lead wire so as to be electrically coupled thereto, and which is soldered to the circuitry.
Typically, the lead wire includes an MP35N lead wire, a platinum/iridium lead wire, or a wire including 1-60% iron by weight.
In a preferred embodiment, the connector includes a hollow tube, wherein a portion of the lead wire is disposed within the hollow tube, and wherein the hollow tube is crimped to the portion of the lead wire. Typically, but not necessarily, the connector includes stainless steel.
For some applications, the circuitry is adapted to be implanted in the patient. For other applications, the circuitry is adapted to be incorporated in a catheter.
In a preferred embodiment, the circuitry includes a sensor, such as a pressure sensor, a temperature sensor, and/or a chemical sensor. Alternatively or additionally, the sensor includes an electrode, adapted to sense electrical activity in tissue of the patient where the apparatus is placed. Further alternatively or additionally, the sensor includes a flow sensor, adapted to sense a flow of blood in a vicinity of the apparatus.
For some applications, the circuitry includes an active element, such as a stimulating electrode, a light source adapted to facilitate photodynamic therapy, an electroactive polymer, and/or a mechanical actuator.
There is also provided, in accordance with a preferred embodiment of the present invention, apparatus which is adapted to be placed in a patient, including:
circuitry, which is adapted to be placed in the patient;
a lead wire, selected from the group consisting of: an MP35N lead wire, a platinum/iridium lead wire, and lead wire including 1-60% iron by weight; and
solder, including at least 20% indium by weight, for electrically coupling the lead wire to the circuitry.
In a preferred embodiment, the solder includes at least 50% indium.
There is further provided, in accordance with a preferred embodiment of the present invention, apparatus which is adapted to be placed in a patient, including:
an electronic device;
a gel; and
a hollow casing, one or more holes being disposed in a wall thereof, inside which casing the electronic device and the gel are disposed, the casing being configured to facilitate flow of some of the gel out of the one or more holes when the casing is being filled with the gel.
Preferably, the apparatus includes including a flexible covering, adapted to fit around at least a portion of the hollow casing. In a preferred embodiment, the flexible covering includes a flexible silicon covering.
Typically, the hollow casing includes a rigid hollow casing, e.g., a hollow stainless steel casing.
For some applications, the circuitry includes a pressure sensor, disposed within the casing such that pressure changes at a patient site where the apparatus is placed are conveyed to the pressure transducer via the gel.
In a preferred embodiment, a sensing hole is disposed in the hollow casing, and a substantially non-metallic flexible covering is disposed outside the casing, so as to cover the sensing hole and to convey pressure changes at the patient site through the sensing hole to the gel. The diameter of the sensing hole is typically less than 2 mm.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings, in which: